This invention relates to a data compression system for use with an apparatus for photometrically analyzing a sample. In particular the invention relates to a data compression system using a logarithmic rationing network configured from only two chopper stabilized operational amplifiers.
A clinical instrument adapted to automatically analyze a sample of a patient's body fluid, such as the device sold by E. I. du Pont de Nemours and Company under the trademark ACA, utilizes a beam of incident light having a predetermined intensity which is directed through a container or cuvette in which the sample of a patient's body fluid is contained. A detector is utilized to measure the intensity of the light beam transmitted through the cuvette carrying the sample of body fluid. A second detector is disposed to provide a second electrical signal representative of the incident light directed toward the sample.
The signals representative of the incident and transmitted light are applied to a ratio signal generator and are utilized to generate a signal representative of the light absorbance of the sample. The resulting analog output signal may be converted to a digital representation, if desired.
Because of the wide range of signal magnitudes involved, the ratio signal generator preferably takes the form of a logarithmic ratioing network. Such a network compresses the data to facilitate further handling. A silicon photocell is preferably utilized for each of the detectors. A silicon photocell exhibits minimum thermal drift when it operates into zero volts, i.e., an effective short circuit. Therefore it is preferred that the logarithmic ratioing network be realized using chopper stabilized operational amplifiers which exhibit a low input offset voltage insensitive to temperature. In addition, if digitization is desired, a dual slop analog-to-digital converter is preferred because its high resolution, conversion accuracy and low cost lend itself for this application area.
However, it has been observed that the voltage signal output from a logarithmic ratioing network of the type which utilizes chopper stablized operational amplifiers produces a periodic noise signal whose average value is not equal to zero. This noise component distorts the output voltage signal to an extent such that applying the output signal to a dual slop analog-to-digital converter the noise component imparts an undesirable error factor into the conversion process which results in an erroneous digital representation of the ratio of the detected light intensities.
Accordingly, it is believed advantageous to provide a logarithmic ratioing network utilizing chopper stabilized operational amplifiers able to produce an output signal representative of the ratio of detected light intensities in which the average value of the noise component of the output signal is zero. Such an output signal may be used in conjunction with a dual slope analog-to-digital converter to give a true digital representation of the ratio.